Molecular genetic analysis of Drosophila development has shown the complexity, diversity, and relatedness of developmental mechanisms in higher organisms. By revealing the kinds of phenomena that underlie human development, study of this model organism provides invaluable insights for improving human health. The following proposal is based on the understanding and genetic tools developed over years of study of the gene Sex-lethal (Sxl), the master regulator of Drosophila sex determination and X-chromosome dosage compensation. The proposal is designed not only to extend specific knowledge of sex determination and dosage compensation, but also to expand our general understanding of the relationship between gene regulation in germ cells and that in somatic cells, the molecular nature of host-parasite interactions in arthropods, the control of behavior by genes, and the unique character of extremely early expressed genes that allows them to be expressed when nearly all other genes are silent. All the experiments proposed rely heavily on the enormous power of forward genetics in this model system. There are three general aims in this proposal, all involving Sxl and all designed to synergize. First, an understanding is sought of how and why Sxl functions in germ cells and how the obligate intracellular parasitic bacterium Wolbachia pipientis interacts with it in that cell type. Wolbachia is a ubiquitous arthropod parasite with health relevance (in River Blindness) that is notorious for manipulating the reproductive biology of its hosts. It may be an important factor driving evolution. This parasite has not previously been amenable to study in a model genetic system like Drosophila melanogaster. The germline functioning of Sxl differs remarkably from its functioning in the soma, which is better understood. The germline studies proposed are relevant to the mechanism of meiosis, stem-cell behavior, cell-cell signaling, gene mutation, and pleiotropy. Second, experiments are proposed to understand a newly discovered branch in the somatic sex-determination gene hierarchy that controls an important aspect of female behavior, namely egg-laying. Finally, studies of the set of genes that act additively to generate the primary sex-determination signal in the fly will be continued to understand how these genes are expressed so much earlier than other genes and whether they are as unique as they seem in this regard.